1. Field of the Invention
The present invention relates generally to the field of transmission and control of optical signals. More particularly, the invention pertains to an electrically controlled apparatus for high speed switching of optical signals. The switching apparatus is especially suitable for applications where the optical signals are transmitted by multimode optical fibers.
2. Discussion of the Prior Art
Until recently, fast optical switching for multimode fiber optic communication and sensor networks was not available. Prior art opto-mechanical switches are known in which the free end of a optical fiber is movable from one contact to another. At each contact, the movable end is aligned with the fixed end of another optical fiber. Such opto-mechanical switches operate at slow speeds (on the order of a few milliseconds) which severely limits their utility. Moreover, repeated flexing of the movable free end of the optical fiber tends to induce fatigue and ultimately results in breakage of the fiber.
By contrast, a fully optical switch is not subject to fatigue and breakage and can operate at much faster speeds.
Systems which use optical switches are able to operate reliably in environments where electronically switched systems have proved to be unreliable because the optical signals transmitted along the optical fibers are not perturbed by electromagnetic interference. Moreover, optical switches are more directly compatible with optical fiber networks than electronic switches because optical fibers can be directly connected to optical switches without requiring converters to change the optical signals to electrical signals.
One example of a recently developed, fully optical switch may be seen in U.S. Pat. No. 4,919,522 to Nelson, granted Apr. 24, 1990, and assigned to the same assignee as the present invention, the specification of which is incorporated herein by reference. That electrically controlled optical switch uses an electro-optic crystal of the kind having two independent sets of fast and slow optical axes in the crystal. One set of fast and slow axes is sensitive to a first electric field extending through the crystal in one direction along a crystal direction and the other set of fast and slow axes is angularly disposed with respect to the first set and is sensitive to a second electric field extending through the crystal in a direction orthogonal to the first electric field.
That optical switch also uses a rectangular parallelepiped crystal with electrodes formed on the four opposing sides of the crystal. The activation of one opposed pair of electrodes causes light to pass through one set of optical axes, while activation of the other pair of opposed electrodes causes light to pass exclusively through the other set of optical axes in the crystal.
Although that prior art switch operates well as a fast optical switch, it requires a crystal having two sets of well-defined fast and slow optical axes in the crystal. Additionally, that switch has an inherent loss of one half of the input light intensity due to the polarizing optics required. This insertion loss reduce the overall light transmission through the switch which decreases the number of switches that may be cascaded in a system.
Therefore, an object of the present invention is to provide an electrically controlled optical switch that operates at a switching speed faster than one microsecond.
Another object of the present invention is to provide an electrically controlled optical switch which only requires a single optical path through a switching element.
A further object of the present invention is to provide an electrically controlled optical switch having lower input light loss than prior art switches.